A fuel cell is an electrochemical reaction device capable of converting chemical energy into electrical energy. Without the restriction of the Carnot cycle, theoretically, the energy conversion efficiency of a fuel cell is higher than that of an internal combustion engine (the maximum energy conversion efficiency of a fuel cell can be 80% or more, and generally, the energy conversion efficiency of a fuel cell is not less than 50%). Moreover, fuel cells have many advantages such as zero emissions, no mechanical noise, etc. Accordingly, fuel cells are popular in the military and civilian fields. According to different electrolytes used in fuel cells, the fuel cells can be classified into five types: alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC) and proton exchange membrane fuel cell (PEMFC). Among them, solid polymer membrane is used as the electrolyte in PEMFC, so it has the advantages of simple structure, low working temperature and high energy conversion efficiency. Therefore, PEMFC is granted with the exceptional edge in mobile power source. Reportedly, at present, Germany and France have developed PEMFC-powered submarines, and several of the world's top automobile companies, such as Toyota Motor Corporation of Japan, have developed PEMFC-powered fuel cell electric vehicles (abbreviated as FCEV or FCV) which have been available for mass production. As an important mobile power source, PEMFC has a good development prospect.
Each individual PEMFC cell consists of two plates (one anode plate and one cathode plate) and a membrane electrode sandwiched between the two plates. The membrane electrode is composed of an anode catalyst, a proton exchange membrane, and a cathode catalyst which are formed together. A gas diffusion layer (GDL) is usually disposed between the anode plate and the membrane electrode, and between the membrane electrode and the cathode plate. The gas diffusion layer is usually made of a gas-permeable carbon paper or carbon cloth. In some disclosures, the gas diffusion layer is a part of the membrane electrode, while in other disclosures the gas diffusion layer is a separate component in the PEMFC. The anode plate of the PEMFC is provided with a fuel flow channel, which is a place where fuel (energetic compounds existing in the state of gas or liquid at normal temperature and pressure, such as hydrogen or methanol) flows and is transported, and the fuel is transported to the anode catalyst via the fuel flow channel. The cathode plate of the PEMFC is provided with an oxidant flow channel, which is a place where the oxidant (usually oxygen or air) flows and is transported, and the oxidant is transported to the cathode catalyst via the oxidant flow channel. By means of the fuel flow channel and the oxidant flow channel, the fuel and oxidant can be continuously transported into the fuel cell so that the fuel cell can continuously output electrical energy.
The structure of Alkaline fuel cell (AFC) is similar to that of the proton exchange membrane fuel cell. Alkaline fuel cell is mainly different from the proton exchange membrane fuel cell in that the electrolyte is different, the fuel flow channel is disposed on its cathode plate, and the oxidant flow channel is disposed on its anode plate. Generally, in order to avoid confusion, the fuel cell plate provided with a fuel flow channel, regardless of whether it is a cathode plate or an anode plate, is collectively referred to as a fuel flow field plate. Similarly, the fuel cell plate provided with an oxidant flow channel, regardless of whether it is a cathode plate or an anode plate, is collectively referred to as an oxidant flow field plate.
In order to improve the total generated power of the fuel cell, a plurality of individual cells are usually connected in series to form a fuel cell stack. In the fuel cell stack, except for the two individual cells at the outermost side, the fuel flow field plates of the individual cells located inside the fuel cell stack are closely appressed with the oxidant flow field plates of the adjacent individual cells. If the fuel flow field plates and the oxidant flow field plates that are appressed with each other are fixedly connected to each other to form an individual component, the structure of the fuel cell stack can be simplified and the reliability of the operation of the fuel cell stack can be improved. Further, the separate component formed by the combination of fuel flow field plate and oxidant flow field plate is called a bipolar plate.
The bipolar plate is one of the critical components of fuel cell stacks, which can realize various functions in the fuel cell stack, such as supporting the membrane electrode assembly, distributing the reaction gas, transmitting current, conducting heat, and discharging the reaction product i.e. water, etc. At the current technical level, the manufacturing cost of the bipolar plate accounts for about half of the total manufacturing cost of the entire fuel cell stack.
Both of the fuel flow field plate and the oxidant flow field plate constituting the bipolar plate are in typical plate-like shapes (i.e. the length and width are much greater than the thickness). Each fuel flow field plate has two opposite surfaces, one surface contains a reference surface and a fuel flow channel (here, referred to as a first surface). Another surface contains a reference surface, and in the fuel cell stack, the reference surface is in contact with the oxidant flow field plate (here, referred to as the second surface). Similarly, each oxidant flow field plate also has two opposite surfaces, one surface contains a reference surface and an oxidant flow channel (here, referred to as the third surface). Another surface contains a reference surface, and, in the fuel cell stack, the reference surface is in contact with the fuel flow field plate (here, referred to as the fourth surface). The reference surface described herein refers to a specific part of the fuel flow field plate or the oxidant flow field plate. If the length direction of the bipolar plate is regarded as an X axis, the width direction of the bipolar plate is regarded as a Y axis, and the thickness direction of the bipolar plate is regarded as a Z axis, then the reference surfaces of the fuel flow field plate and oxidant flow field plate are parallel to the plane formed by the X axis and Y axis and perpendicular to the Z axis. Moreover, the distance between the reference surface on the first surface and the reference surface on the second surface is equal to the thickness of the fuel flow field plate. The distance between the reference surface on the third surface and the reference surface on the fourth surface is equal to the thickness of the oxidant flow field plate. The fuel flow channel, the oxidant flow channel, and the coolant flow channel are grooves formed in the Z-axis direction with respect to the reference surface on respective surface. When assembling the bipolar plates, the second surface of the fuel flow field plate and the fourth surface of the oxidant flow field plate are closely appressed with each other and the two plates are fixedly connected by welding, adhesion or other connection methods.
In the fuel cell stacks, most of the chemical energy carried by the fuel is converted into electrical energy, while a certain part of the chemical energy is converted into heat energy. Therefore, in view of a fuel cell stack having a large generated power, the heating power is also rather large. As a result, the cooling of the fuel cell stack becomes a noticeable problem. If the cooling intensity is insufficient, the temperature of the fuel cell stack will increase continuously, which will cause burn-out of the fuel cell stack. In order to ensure that the fuel cell stack generates electricity continuously at a relatively constant temperature, it is necessary to introduce a coolant into the stack to strengthen the cooling of the stack. Currently, the commonly adopted method is to provide a coolant flow channel on the second surface of the fuel flow field plate and/or the fourth surface of the oxidant flow field plate, and to introduce the coolant into the interlayer of the bipolar plate to carry the heat energy generated by the reaction out of the fuel cell stack.
For such kind of fuel cell stack with a large generated power, since the bipolar plate is required to be provided with the coolant flow channel, the bipolar plate is usually designed with a “three-in, three-out” structure. Namely, a fuel flow channel is disposed on one surface of the bipolar plate, an oxidant flow channel is disposed on another surface of the bipolar plate, a coolant flow channel is disposed at the middle of the bipolar plate, and a fuel inlet channel, a fuel outlet channel, an oxidant inlet channel, an oxidant outlet channel, a coolant inlet channel, and a coolant outlet channel are disposed at the edges of the bipolar plate. It should be noted that in some disclosures, the inlet/outlet channels are also referred to as manifolds.
In addition, the bipolar plates must be made of conductive materials due to the requirement of current transmitting. Currently, there are three main types of materials used for making bipolar plates, i.e. graphite materials, composite materials, and metal materials. Graphite bipolar plates have good electrical conductivity and high corrosion resistance. However, due to the high brittleness and poor mechanical properties of the graphite material, the manufacturing cost of the graphite bipolar plates is relatively high. The main raw materials of the composite bipolar plate include graphite powder and resin, and the composite bipolar plate is manufactured by means of molding, etc. So, it has a relatively low manufacturing cost while having problems such as low conductivity and poor airtightness. The bipolar plates formed by sheet metal (such as titanium plate, stainless steel plate or aluminum plate with a thickness of 0.1-0.2 mm, etc.) subjected to stamping and then to the processes of welding, anti-corrosion treatment, etc., and have high strength, good performances of electrical conductivity and thermal conductivity, and a relatively low manufacturing cost, which is the mainstream method for manufacturing bipolar plates, currently.
The bipolar plate with a “three-in, three-out” structure formed by sheet metal subjected to stamping has two noticeable characteristics. First, the fuel flow field plate and the oxidant flow field plate are both in a corrugated shape. Namely, the coolant flow channel is simultaneously formed following the formation of the fuel flow channel or the oxidant flow channel. If the length direction of one polar plate is regarded as an X axis, the width direction of the polar plate is regarded as a Y axis, and the thickness direction of the polar plate is regarded as a Z axis, for a specific point on the polar plate (i.e., a point that corresponds to a specific X-axis coordinate and a specific Y-axis coordinate), if the first surface where the point is located is the reference surface, then the second surface where the point is located is the bottom of the coolant flow channel; if the first surface where the point is located is the bottom of the fuel flow channel, then the second surface where the point is located is the reference surface; if the third surface where the point is located is the reference surface, then the fourth surface where the point is located is the bottom of the coolant flow channel; if the third surface where the point is located is the bottom of the oxidant flow channel, then the fourth surface where the point is located is the reference surface. Second, both of the second surface of the fuel flow field plate and the fourth surface of the oxidant flow field plate are provided with the coolant flow channel. Moreover, except for some special parts, the coolant flow channel on the second surface of the fuel flow field plate and the coolant flow channel on the fourth surface of the oxidant flow field plate are aligned and combined with each other in a snap-fit manner (i.e., the projections of the two coolant flow channels in the plane formed by the X axis and the Y axis are overlapped with each other) to form a complete coolant flow channel when assembling the bipolar plate. Normally, the coolant flow channel on the second surface of the fuel flow field plate and the coolant flow channel on the fourth surface of the oxidant flow field plate each account for a half of the coolant flow channel.
The above two characteristics cause a problem that in the fuel cell stack, the fuel inlet channel, the various fuel flow channels and the fuel outlet channel must form a closed space to avoid fuel leakage and mixing of fuel and oxidant because the fuel leakage and mixing of fuel and oxidant are dangerous and can cause fire or even an explosion under the action of electrode catalyst. Therefore, the various fuel flow channels must be isolated from the coolant inlet channel and the coolant outlet channel. Correspondingly, the coolant flow channel located on the back surface (second surface) of the fuel flow channel and between the two fuel flow channels is naturally isolated from the coolant inlet channel and the coolant outlet channel. Unless specific measures are taken to interconnect these coolant flow channels with the coolant inlet channels and the coolant outlet channels, these coolant flow channels will form separated closed spaces where the coolant cannot flow in and out. Therefore, when the bipolar plate having the “three-in, three-out” structure is formed by the metal sheet subjected to stamping, how to interconnect various coolant flow channels with the coolant inlet channel and the coolant outlet channel to improve the cooling intensity of the fuel cell stack and the temperature control system of the fuel cell stack is of great significance. So far, however, few studies focus on this issue.
In addition, when a bipolar plate having a “three-in, three-out” structure is formed by the metal sheet subjected to stamping, the design solution available now also has problems in the layout of various channels. According to the design solution available now, the coolant inlet channel and the coolant outlet channel are respectively disposed at intermediate positions of both ends of the bipolar plate. Since the coolant is a fluid, and the path with the minimum flow resistance during its flow will always have the maximum flow rate. Therefore, when the coolant flows in the interlayer of the bipolar plate, the cooling intensity of the middle part of the bipolar plate will be significantly higher than that of both sides. The two sides of the bipolar plate, which accounts for about two-thirds of the total area of the bipolar plate, are “dead area” for the coolant and thus cannot be sufficiently cooled.